Phosphoric-acid recovery



highly heated charge.

U NM .21

EDOUARD WILLIAMS, 0F BREW PANY, OF NEW No Drawing.

l,'il4,68

DU BOIS IMATHEY, OI CRANFORD, NEW JERSEY, ANI'J' AUDLEY OSCAR STER,FLORIDA, ASSIGNORS TO AMERICAN CYANAMID COM- YORK, N. Y., A CORPORATIONOF MAINE.

.EHOSPHORIC-ACID RECOVERY.

This invention relates to a method of producing phosphoric acid in casesin which either the element its compounds,

elf or one of its volatile is liberated and expelled from a It refersmore particularly to treatments of a nature carried out in a shaftfurnace heat the mass.

employing electrodes to In the present embodiment of the invention,

the preliminary hea combustion of fuel,

ting is effected by the elemental phosphorus,

etc. in the shaft of the furnace, and the final or reaction temperatureis attained by electrical means in the furnace pot.

furnace is of the arc charging shaft.

The electric type and has a vertical The present invention refersparticularly to a new method of increasing the pliability and flow ofthe charge in obtaining a regulated the vertical shaft, and

at the same time affording an easy method of escape for the upwardrefers to a process moving gases. It thus increasing the porosity andpermeability of the charge in the shaft,

and at the same time movement-.-

retarding its downward The object of this invention is to so regulatethe feeding in the shaft of the furnace that the materials themselveswill not' block and clog the passage, and on the other hand will notenter the pot too precipitously. With these and other objec consists inthe various steps and combina- P tion of steps constitu ts in view theinvention ting the process, and in the employment of a new materialcapable of improved results.

' bringing about simultaneously two or more all as will be more fullyhereinafter described and particularly pointed out in the claims As anexample of the process, 1000 parts of phosphate rock, 400 parts ofsilica, parts 0 coke and 300 parts by welght of wood are taken tocompose the fed into the shaft in charge. The material is the furnacethrough the charging device, and as it descends it meets the upwardrising gases, in contact with the charge which are burned by theintroduction of air in the lower part of the shaft. After their escapefrom the furnace the gases carrying the phosphoric anhydride are treatedin any of the various methods found suitable for the collection of thephosphoric acid. Among these may be mentioned the absorption in towers,or the elec trical precipitation by f shaft and arch over.'

Application filed September 6, 1923. Serial- No. 661,311.

means of the Cottrell process. At the bottom of the shaft is the pot ofthe furnacein which the electrodes are located, requisite temperature isattained, and the phosphorus is liberated from the molten mass. When allthe phosphorus is expelled the slag is discharged, from the furnace. Ithas been found advisable to maintain the reaction by an almostcontinuous withdrawal of. the residual slag.

Before entering the furnace the charge material may be treated invarious ways. The roclr, and silica may receive a preliminary sinteringto avoid shaft choking, which is very common where fine powders areused. In this case the sintered material and the wood are introducedinto the furnace. Another and more improved method consists inintroducing the silica in the form of sandstone, and dispensing withthepreliminary clinkering. TlllS is a very economical method. It does awaywith the necessity of a preliminary sintering action, lessens the amountof fuel required and very materially cuts down the with other fuels,such as coke, coal, etc. may

be varied at will, and in fact wood alone may replace all other fuels ifso desired.

In the present invention the addition of wood is an extremely importantfeature and this whether added in small or large blocks, lays a fourfoldpart in the process. A difficulty that is always met with in operatingshaft furnaces, whether of the ordinary or the electrode type, is theproper feeding of the charge through the shaft. There is always atendency to either choke or to precipitate the fresh material into thereaction zone at a too rapid rate, or for the mass to block in the Inthe case of electric shaft furnaces the difficulties resulting from animproper feeding of the charge, are much greater than in an ordinaryshaft furnace, because a sudden accumulation of fresh material betweenthe electrodes will cut ofi the arc.

When the wood added is of a size that will properly bolster up the massabove it and through wedging, support the weight until the strength ofthe wood is lost by carbonization, the downward movement is regulatedand the vertical column of fresh materialcdoes not bear too heavily uponthe molten mass in the bath. Another important feature consists in theopening up of gas passages by shrinkage of the wood in changing tocharcoal. In converting the phosphorus vapor into phosphoric anhydride,air is injected into the shaft just above the molten mass, and this aidsin the shrinkage of the carbonized wood.

When the carbonized wood finally reaches the molten bath it is usuallyreduced to the condition of small chunks, and owing to the remarkablechemical activity of charcoal, it is more eflicient in the reductionofphosphorus compounds than any other formof carbon. As the relativeproportion of Wood to other reducing agents such as coke, coal,

: etc., may be varied at will, it is possible to obtain almost anydesired condition of the charging mass.

Although originally processes were suggested in which the chargeconsisted exclusively of phosphate rock and sand, these methods werelater abandoned owing to the imperfect volatilization of the phosphoricanhydride, and the length of time required to effect the reaction.Theoretically phosphoric anhydride should be liberated on fusion withsilica, but in practise it is found that complex side reactions are setup, which hold back the phosphoric anhydride, and in addition theviscosity of the molten charge hinders the liberation of the volatileanhydride. If new a suflicient amount of a reducing agent is added thephosphorus compounds will all be reduced to' elemental phosphorus, whichin vapor form is easily expelled from the bath. This reduction entailsthe use of a very considerable amount of reducing agent, usually in theform of wood and charcoal coke or fine coal, and as the phosphorus mustbe completely oxidized before it is collected, it is desired to recoveras large a proportion of the energy supplied in the reducing agent as ispossible. Consequently the gases are brought into intimate contact withthe incoming charge, and air introduced, and the gases burned in contactwith the charge. Theoretically the oxidation of the phosphorus shouldfurnish to the incoming charge all the heat expended in reducing thephosphorus compounds, but the Various side reactions and radiations makethis impossible. To overcome as far as possible the loss consequent uponthe oxidation and burning away of the Wood by the injected air, thelatter is regulated so that the combustion and the flame area ismaintained close to the bath, and seldom reachesfar up in the shaft. Thegaseous products of combustion thus pass through the maximum length ofcharge in the shaft, and finally come in contact with the cold chargeimmediately after its introduction. This conserves the heat to thegreatest degree possible.

Among the many advantages in the employment of wood as a regulating andreducing means, are its ready availability and rela- 'tively. small costin regions where extensive water-powers exist, and the fact that allshapes, sizes, and varieties may be used.

It is obvious that those skilled in the art may vary the details of theprocess, and the nature of the product without departing from the spiritof the invention, and therefore we do not wish to be limited to theabove disclosures except as may be required by the claims.

1. A method of manufacturing phosphoric acid which comprises heating inan electric furnace an inorganic material capable of liberatingphosphorus at elevated temperatures in the presence of sticks ofcarbonized wood, substantially as described.

2. A method of manufacturing phosphoric acid which comprises heating inan electric furnace a material capable of liber ating phosphorus bymeans of reducing agents other than wood, and-sticks of wood, andoxidizing the expelled phosphorus, substantially as described.

3. A method of manufacturing phos-' phoric acid which comprises heatingin an electric furnace of the shaft type, a mixture of phosphate rockand sand, mixed with sticks-of wood, and so regulating the feeding ofthe bath that the wood will be carbonized by the heat generated in thefurnace pot, in addition to that resulting from the combustion of theescaping gases, and oxidizing the volatilized phosphorus, substantiallyas described.

4. A method of manufacturing phosphoric acid which comprises heating inan electric furnace of the shaft type, a clinkered mass of phosphaterock, sand and coke, mixed with sticks of wood, and so regulating thefeeding of the bath that the wood will be carbonized by the heatgenerated in the furnace in addition to that resulting from thecombustion of the escaping gases; and oxidizing the volatilizedphosphorus, substantially as described.

5. A method of manufacturing phosphoric acid which comprises heating inan electric furnace phosphate rock, sand and chunks of wood, and soregulating the descent of the charge into the molten bath that the woodwill become carbonized before entering the molten bath, and oxidizingthe expelled phosphorus substantially as de scribed.

6. A method of manufacturing phosphoric acid which comprises heating inan electric furnace of the shaft type, a mass of material capable inpresence of a reducing agent of expelling phosphorus at elevatedtemperatures, accompanied by blocks of wood, and so regulating thecombustion of the rising gases through the introduction of air, that thewood will be slowly and progressivelycarbonized, and in shrinking willaid the escape of the rising gases, and the downward movement of thecharge, and oxidizing the liberated phorphorus, substantially asdescribed.

7. A step in the method of manufacturing phosphoric acid which comprisescharging an electric shaft furnace with a mixture of phosphate rock,sand, and coke, in the form of clinker, accompanied by blocks of wood insize sufficient to support and retard the downward movement of thecharge in the shaft, and on charring allow the upward rising gases topenetrate the mass, and the charge itself to slowly be released,substantially as described. a

8. In a method of manufacture of phosphoric acid from phosphate rock andsilicious material, in an electric furnace, the step which comprisesadding to the charge a substance capable of equalizing the downwardmovement of the charge in the stack, increasing the permeability of thecharge to the upward movement of the escaping gases, and capable in themolten bath of reducing compounds of phosphorus to the elemental state;substantially as described.

9. In a method of manufacture of phosphoricacid from phosphate rock andsilicious material in an electric furnace, the step c which comprisesadding to the charge a substance capable of decomposing and formingcharcoal at low temperatures, and shrinking during the decomposition,substantially as described.

10. In a method of manufacture of phosphoric acid from phosphate rockand silicious material in an electric shaft furnace, the step whichcomprises adding to the charge a substance capable of impeding thedownward movement of the charge in the shaft, before decomposition, andlater aiding the downward movement of the charge during decomposition,and finally aiding the liberation of the phosphorus in the molten bath,substantially as described.

11. A method of manufacturing phosphoric acid which comprises mixingwood with a mass capable of liberating phosphorus at elevatedtemperatures, heating the mixture, and oxidizing the liberatedphosphorus.

12. A method of manufacturing phosphoric acid which comprises mixingsticks of woodwith a clinkered mass of phosphate rock and silicia,heating the mixture in an electric furnace, and oxidizing the escapingphosphorus.

In testimony whereof we aflix our signatures.

EDOUARD DU BOIS MATHEY. AUDLEY OSCAR WILLIAMS.

